This invention relates to an optical memory apparatus capable of performing write-in and read-out operations with a light beam, and more particularly to an optical memory apparatus with a compensation function for the irradiation position of a light beam on a memory medium.
Recently, optical memory apparatus have come to be regarded as suitable for large-scale, high density information storage devices due particularly to the focusing capability of light beams. In such an optical memory apparatus, means for providing light beam access to a desired memory position in an information memory medium is important. A method resorting only to the operation of a light deflector can achieve the highest access speed. However, since a light deflector with a number of deflection points has not as yet been developed, an optical disc memory apparatus or an optical drum memory apparatus in which access to a desired memory position is carried out by the mechanical rotation of a memory medium in combination with the deflection by a light deflector is used in place of a memory apparatus with only a light deflector.
Generally, in an optical memory apparatus, to achieve an inherently high memory density, a light beam is focused on an information memory position with a diameter of the order of several microns, and, as a result, a position deviation of a disc rotary shaft of an optical disc memory apparatus in the direction parallel to the disc plane or a position deviation of a drum rotary shaft in an optical drum memory apparatus in the direction in which said rotary shaft extends will result in an off-track condition between the light beam and an information memory track. This off-track situation has been a serious problem. To solve this problem, a method of compensating for the deviation by detecting the position deviation and negatively feeding back the detected signal to the light deflector, and another method of employing an air bearing with a high precision in a rotating mechanism have been proposed.
An example of a specific prior art method is the optical memory system "UNICON" disclosed in an article titled "Laser Mass Memory System" published in "IEEE TRANSACTIONS ON MAGNETICS", September issue, 1972, Vol. 8, No. 8, pp. 416-420. An example of the latter method is seen in an article titled "Advanced Optical Storage Techniques for Computers" published in "Applied Optics", (1972) Vol. 11, No. 10, pp. 2133-2139.
However, the first method has the disadvantage that very complicated optical and electro-mechanical means are required with the result that the apparatus becomes costly to manufacture and the response speed is comparatively low. The second method also has a disadvantage in that the accuracy is very low. Furthermore, even when a static memory medium is employed without introducing the mechanical rotation of the above-described memory medium, the arrangement of the optical system varies due to vibration resulting from variations of the irradiating position of the light beam. Consequently, it is necessary that the optical system be installed on a massive optical bench. It is, therefore, very difficult to increase the storage density of the optical memory apparatus to fully utilize the focusing capability of a light beam.
It is, therefore, an object of the present invention to provide an optical memory apparatus with a novel compensating mechanism for a light beam irradiating position capable of realizing a high density memory by the use of the focusing capability of a light beam free from the disadvantages of the prior art optical memory apparatus described above.